The following is a discussion of relevant art pertaining to TP53 and RNAi. The discussion is provided only for understanding of the various embodiments of invention that follow. The summary and references cited throughout the specification herein are not an admission that any of the content below is prior art to the claimed invention.
The TP53 tumor suppressor is activated by protein stabilization following genotoxic stress. This activation can be induced by ultraviolet or ionizing radiation as well as a host of DNA-damaging chemotherapeutics such as doxorubicin (adriamycin), cisplatin, and bleomycin. Activation of TP53 leads to cell cycle arrest prior to entry into S phase and/or apoptosis. TP53 activation also initiates a number of DNA repair pathways (Fei and El'Deiry, 2003, Oncogene 22:5774-83). Mutations in TP53, which are present in about 50% of human cancers (Hollstein et al., 1991, Science 253:49-53), result in checkpoint defects and may contribute to uncontrolled cell proliferation, genomic instability, and accumulation of tumorigenic mutations (Prives and Hall, 1999, J. Pathol. 186:112-26). In the clinic, emphasis has been placed on identifying chemotherapeutics that are effective for both TP53-positive tumor cells and TP53-deficient tumor cells (Lowe et al., 1994, Science 266:807-810; Lacroix et al., 2006, Endocrine-Related Cancer 13:293-325; Levesque and Eastman, 2007, Carcinogenesis 28:13-20). Therefore, predicting TP53 pathway status in human tumors will be an important component for selecting an effective cancer therapeutic for a given cancer type.
Although DNA sequencing of TP53 can reveal inactivating mutations, the TP53 pathway can be inactivated by alternative mechanisms. For example, p19(ARF), which is encoded by the INK4a-ARF locus, inhibits cell proliferation by activating TP53 (Sherr et al., 2005, Cold Spring Harbor Symp. Quant. Biol. 70:129-37). Significantly, many human cancers exhibit deletion, silencing, or mutation of the INK4a-ARF locus. Other tumors over-express, or express aberrant splice forms of, MDM2, a key regulator of TP53 stability and transcriptional activity (Levav-Cohen et al., 2005, Growth Factors 23:183-92). TP53 pathway inactivation can also be caused by viral factors such as the human papilloma virus E6 protein, which binds to and targets TP53 for degradation. Therefore, predicting TP53 pathway integrity may not be straightforward in many patient tumors. Miller et al. (2005, PNAS 38:13550-55) developed a gene expression signature to predict TP53 pathway status of cancer patients and presented data showing the importance of TP53 pathway status in predicting clinical breast cancer behavior.
There is growing realization that miRNAs, in addition to functioning as regulators of development, can act as oncogenes and tumor suppressors (Akao et al., 2006, Oncology Reports 16:845-50; Esquela-Kerscher and Slack, 2006, Nature Rev., 6:259-269; He et al., 2005, Nature 435:828-33) and that miRNA expression profiles can, under some circumstances, be used to diagnose and classify human cancers (Lu et al., 2005, Nature 435:834-38; Volinia et al., 2006, PNAS 103:2257-61; Yanaihara et al., 2006, Cancer Cell 9:189-198). Given the significance of TP53 in cancer and the importance of finding clinical biomarkers for TP53 status, there is need to identify RNA transcripts, including miRNAs, that are involved in regulation of the TP53 pathway.